1. Field of the Invention
This invention relates to a vibration/noise control system, and more particularly to a vibration/noise control system which actively controls vibrations and noises generated with a periodicity or a quasi-periodicity from a rotating member and the like, to thereby reduce the vibrations and noises.
2. Prior Art
Recently, vibration/active noise control systems have intensively been developed in various fields of the industry, which are adapted to damp vibrations and noises produced from vibration/noise sources, by the use of an adaptive digital filter (hereinafter referred to as "ADF"), to thereby reduce the vibrations and noises.
These conventional vibration/active noise control systems include a vibration/noise control system proposed by Japanese Patent Application No. 5-86823 filed by the present assignee and U.S. Ser. No. 08/189,912 (now U.S. Pat. No. 5,544,080) corresponding thereto, wherein a sine wave signal having a single repetition period is generated depending on the repetition period of vibrations and noises peculiar to component parts of the vibration/noise source, and the sine wave signal and a delayed sine wave signal which is delayed in phase by a predetermined period relative to the former are input to the ADF.
In the proposed vibration/noise control system, a Wiener filter (hereinafter referred to as "the W filter") of a Finite Impulse Response (FIR) type having two taps (filtering order number) is employed as the ADF, and a rotation signal from a rotating member is detected in the form of a pulse signal whenever the rotating member rotates through a predetermined very small rotating angle (e.g. 3.6.degree.). More specifically, in the proposed vibration/noise control system, a sine wave signal for one repetition period is generated whenever the rotating member rotates one rotation (360 degrees), and the thus generated sine wave signal and a delayed sine wave signal obtained by delaying the sine wave signal in phase by a predetermined period are input to first filter means for executing adaptive control, whereby even with the use of the ADF having two taps, the adaptive control can be achieved, enabling a reduction in the time period required for the product-sum operation to be carried out.
Further, in the proposed vibration/noise control system, the transfer characteristic of a transmission path of vibrations and noises to be controlled is stored in a table incorporated in second filter means, as results of predetermined identification processing carried out beforehand, and the transfer characteristic stored in the second filter means is read out to thereby correct a control signal for canceling the vibrations and noises. Thus, according to the proposed vibration/noise control system, the transfer characteristic which has been once stored into the second filter means is regarded and treated as a fixed characteristic during control operation of the vibration/noise control system.
Vehicles, such as automotive vehicles, in which vibrations and noises are generated with a periodicity or a quasi-periodicity are used to travel under various environments over a long time period, and hence the transfer characteristic of the vibration/noise transmission path changes depending on environments under which the vehicle travels. In particular, when vibration/noise control is carried out for a vehicle in which the engine is mounted on a so-called self-expanding engine mount, there can occur a change in the elasticity of rubber members constituting part of the engine mount due to dependency thereof on the temperature, and/or hardening of the rubber members due to aging, which causes to a change in the transfer characteristic. Further, the transfer characteristic of vibrations and noises within the compartment delicately changes depending on various factors, such as the temperature, the humidity, open/closed states of windows of the vehicle, and seating locations of passengers and the number of the passengers.
In the proposed vibration/noise control system, however, since the transfer characteristic stored in the second filter means is regarded and treated as a fixed characteristic during the vibration/noise control, it is necessary to correct the transfer characteristic for a change in the elasticity of the rubber members due to aging, etc. by means of identification processing on an occasion such as a safety checking of the vehicle. Further, it is also necessary to correct the transfer characteristic for a change in the temperature by means of a temperature sensor. However, this further requires the provision of a memory having a large capacity and temperature sensors for each rubber member, etc., resulting in a complicated identification operation as well as an increase in the number of component parts and an increase in the labor and time.
Therefore, to carry out highly accurate vibration/noise control in dependence on aging and environmental change, it is desirable that correction of the transfer characteristic of the vibration/noise transmission path should be carried out during the adaptive control. To this end, an active noise control system has been proposed, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 5-265468, wherein an identifying sound corresponding to a background noise level within a predetermined space to be subjected to noise control is generated and output, and the transfer characteristic of the noise transmission path is determined based on the identifying sound and a residual noise at a predetermined location within the predetermined space, to thereby identify the transfer characteristic of the noise transmission path during execution of the noise control.
According to the proposed active noise control system, the identifying sound generated is lower in level by a predetermined amount than the background noise so that the transfer characteristic of the noise transmission path can be identified without the identifying sound being sensed by the passenger(s).
In the proposed active noise control system, to obtain highly accurate identification results, the identifying sound is required to have a good S/N ratio.
If the identifying sound is set to a higher level to increase the S/N ratio, the identifying sound is sensed by the passenger(s), to thereby give an uncomfortable feeling to the passenger(s). Therefore, the identifying sound should be set to a level as small as possible. In other words, when the proposed active noise control system is applied to an automotive vehicle, the level of the identifying sound can be increased only to a limited degree. In addition, the noise level within the compartment is large due to road noises and the like during travel of the vehicle, so that it is difficult to maintain the S/N ratio at a satisfactory level. Thus, the proposed active noise control system can achieve only a limited accuracy of identification results, and hence is incapable of performing proper noise control in response to aging change and environmental change.
Moreover, the proposed active noise control system employs an ADF having many taps, and hence requires a long time period to identify the transfer characteristic.